Publisher Summary The problem of the rise of water in tall plants is as old as the science of plant physiology. This chapter considers the cohesion theory, which is the best formulation to explain how water can get to the top of tall trees and vines. The cohesion theory assumes that diffusion of water from the noncollapsible xylem elements in contact with the leaf cells creates a state of tension within the water columns in the xylem vessels. This tension is possible because of the cohesion of water molecules and their adhesion to the hydrophilic walls of the xylem elements. Tension in the water columns is assumed to lift water from the roots to the leaves, in addition to reducing the potential energy of the water in the root xylem tissue until water diffuses from the soil into the root during absorption of the water. The cohesion theory assumes continuity of water columns, laterally and vertically, in the conducting elements of the xylem tissue. These water columns ultimately are placed under tensile strain. But widespread rupture is believed not to occur in the water columns under tensile strain owing to the purported cohesive properties of water when entrapped in small capillaries.